This invention relates to pressure filters.
In recent years, there has been developed a type of pressure filter, conveniently called a tube pressure filter, which is capable of operating at high pressures, that is to say at pressures in excess of 500 pounds per sq. inch (35.2 kg/cm.sup.2). Tube pressure filters can be used in the filtration of a wide range of materials, including aqueous slurries of particulate solids, such as, for example, mineral pigments, insoluble oxides and hydroxides of metals, refractories and coal dust. Whilst tube pressure filters are of particular value in the pressure filtration of aqueous slurries, such as clay slurries, containing a relatively large proportion of fine particles (since such slurries often cannot be so readily dewatered using other known types of filtration equipment), they may also be used in the pressure filtration of slurries of other particulate solids, including those of a coarse, granular or fibrous nature such as, for example, slurries of particulate solids containing particles larger than 0.5 mm, or waste water from paper mills and from various operations in the textile industry.
Several kinds of tube pressure filter have been described, and details of the construction and operation of some of these are given, for example in British Patent Specification Nos. 907,485; 1,240,465; 1,240,466; 1,271,494; 1,317,887; 1,336,856; 1,340,858; 1,351,142; 1,351,943; 1,355,553; 1,362,655 and 1,386,256; and in the U.S. Pat. No. 3,900,403. Most tube pressure filters comprise a pair of generally coaxial inner and outer tubular assemblies which are arranged one within the other and define between them a chamber of annular or substantially annular cross-section and which are adapted to be supported in a generally upright position, an impermeable elastic sleeve secured to the outer one of said pair of tubular assemblies so as to divide said chamber into non-intercommunicating inner and outer compartments, a filter element disposed around and supported by the inner tubular assembly, a slurry inlet for feeding a slurry to be pressure filtered to the inner compartment, a hydraulic fluid inlet for feeding a hydraulic fluid to the outer compartment and a hydraulic fluid outlet for withdrawing hydraulic fluid from the outer compartment, filtrate discharge means for use in effecting the discharge of filtrate which has passed through the filter element and through apertures in the inner tubular assembly, and means for use in effecting the discharge of solid material retained in said inner compartment.
The filter element of a tube pressure filter usually comprises a sleeve of filter cloth material, advantageously supported on a wire mesh screen, which fits snugly around the inner tubular assembly of the tube pressure filter.
Usually, the means for use in effecting the discharge of solid material retained in said inner compartment comprises means for displacing the tubular assemblies axially relative to one another between two operating positions. In one of the operating positions, a slurry to be dewatered can be fed through the slurry inlet to fill the inner compartment (which is otherwise closed) and thereafter the slurry can be dewatered by supplying hydraulic fluid under pressure to the outer compartment; and in the other operating position, the outer compartment is empty, the hydraulic fluid having been withdrawn from the outer compartment, and the inner compartment is open to enable the solid material retained in the inner compartment after the dewatering process has been completed, to be discharged from the tube pressure filter. During the dewatering of the slurry in the inner compartment, the liquid component of the slurry is forced through the filter element (and through apertures in the inner tubular assembly) while the particulate solid component of the slurry is retained on the filter element.
In the known tube pressure filters, a slurry to be dewatered has generally been introduced into the inner compartment of the tube pressure filter via a slurry inlet which is formed in the inner tubular assembly and is in communication with the inner compartment of the tube pressure filter throughout the period when pressure is being applied to the slurry in the inner compartment, by means of the hydraulic fluid in the outer compartment, to dewater the same. The slurry inlet usually takes the form of a very narrow orifice or of an orifice the mouth of which can be closed by means of a device, for example an elastic ring, acting as a non-return valve. If the orifice is neither narrow nor capable of being closed by means of a non-return valve, there exists a risk that the impermeable elastic sleeve which separates the inner and outer compartments will be extruded into the orifice under the action of the very high hydraulic pressures which are exerted on the impermeable elastic sleeve during the dewatering of the slurry in the inner compartment. Unfortunately, it is found that when the feed material to be dewatered is a slurry of a particulate solid which is a coarse, granular or fibrous material, the particulate solid tends to block the narrow orifice or to cause damage to the non-return valve which is used to close the orifice. A more complicated form of slurry inlet has been proposed in British Patent Specification No. 907,485 which discloses some embodiments of tube pressure filter in which the inner compartment is isolated from the slurry inlet during the dewatering process with the slurry inlet comprising a poppet valve system which is incorporated in either the inner or the outer tubular assembly. However, these embodiments require the poppet valve system to operate immersed in the feed slurry so that fouling of the poppet valve and/or of its valve seat by solid material in the feed slurry in likely to occur, especially when the feed slurry contains a coarse, granular or fibrous material, with consequential excessive wear of the poppet valve and its valve seat.